The peak figure of merit (ZT) of GeTe-based thermoelectric (TE) materials is typically attained in the high-temperature cubic phase, where the inevitable phase transition raises concerns over interfacial instability during operation. Therefore, developing high-performance rhombohedral GeTe below the phase transition temperature represents a more viable path toward practical applications. Herein, we propose a facile nanocomposite strategy to enhance the TE performance of rhombohedral GeTe by incorporating high-modulus TiB2 nanoparticles into Ge0.94Bi0.05Te matrix. We demonstrate that the nanoparticle-induced interfacial constraint effect contributes to increasing longitudinal elastic modulus and decreasing equivalent deformation potential, accounting for improved carrier mobility. Additionally, these TiB2 inclusions form heterogeneous interfaces that promote charge depletion and generate substantial thermal resistance, concurrently suppressing the heat transfer by carriers and phonons. Consequently, an extraordinary ZT of 2.66 at 613 K and a superior average ZT of 1.29 (300 ~ 613 K) are obtained in the rhombohedral GeTe-based composite. This work shows a paradigm for synergistically optimizing the electrical and thermal transports of emerging TE systems with nanoinclusions.

Band convergence has been utilized as an effective strategy to enhance the Seebeck coefficient, typically by improving the energy dependence of carrier density near the Fermi level. In contrast, the energy dependence of carrier mobility, which can also enhance the Seebeck coefficient, has attracted less attention. Here, we show that the Seebeck coefficient can be improved when the additional converging band contributes highly mobile carriers, even if they have minimal impact on carrier density. The high-mobility carriers enhance the net diffusion flux, promoting carrier accumulation and amplifying the Seebeck voltage. This mobility-driven enhancement exhibits a hump-like feature in the Pisarenko plot, which becomes flatter and shifts toward lower carrier concentrations as the bands approach. The transport properties of p-type rhombohedral GeTe, as a potential case, were experimentally examined. This work deepens the understanding of band convergence-induced Seebeck coefficient enhancement in thermoelectric materials.

Cubic‐phase GeMnTe2 shows high potential to replace state‐of‐the‐art rhombohedral GeTe for medium temperature thermoelectric application owing to its lower cost. The high structural symmetry can also suppress phase transition during service and provides a superior platform for further band and microstructural engineering. Through Sb2Te3 alloying and Pb substitution, this study realizes a superior peak figure of merit of ≈1.5 at 773 K and a remarkable average figure of merit of ≈0.96 at 323–823 K. Sb2Te3 alloying successfully generates high‐density localized van der Waals (vdW) gaps which are able to scattering low‐frequency phonons effectively for reduced lattice conductivity; meanwhile, it also enlarges the valence band degeneracy for enhanced power factor. Pb substitution further reduces the hole concentration to an optimal level. The achievements in this work well reveal the efficacy of construing localized vdW gaps in improving matrix material's thermoelectric performance, thus might shed light on other cubic or pseudo‐cubic thermoelectric systems.

Symmetry-guided crystal structure design enhances average zT and mechanical properties in rhombohedral GeTe

Enhanced thermoelectric performance of rhombohedral GeTe by Yb-Bi Co-doping

Conventional piezoelectric materials typically exhibit positive longitudinal piezoelectric coefficients, yet recent studies have identified exceptions with negative piezoelectric responses. Using density functional theory, we demonstrate for the first time that rhombohedral GeTe (r-GeTe) possesses an ultrahigh negative piezoelectric strain coefficient (d33) of -70.87 pC/N, surpassing all previously reported negative piezoelectric materials. This phenomenon arises from the "quasi-layered" structure of r-GeTe, comprising alternating strong and weak bonds, which induces a pronounced negative internal-strain contribution and an exceptionally low elastic constant. We further extend our investigation to other IV-VI rhombohedral materials, identifying GeS, GeSe, and SiTe as promising candidates for ultrahigh negative piezoelectricity. In contrast to prior reports, where negative piezoelectricity stems from a negative clamped-ion term that dominates a small positive internal-strain contribution, our findings propose a new material design strategy for large negative piezoelectricity by introducing a significantly negative internal strain, along with the negative clamped-ion term.

Superior electronic performance due to the highly degenerated Σ valence band (Nv∼12) makes rhombohedral GeTe a promising low-temperature (<600 K) thermoelectric candidate. Minimizing lattice thermal conductivity (κL) is an essential route for enhancing thermoelectric performance, but the temperature-dependent κL, corelated to T-1, makes its reduction difficult at low temperature. In this work, a room-temperature κL of ≈0.55Wm-1-K-1, the lowest ever reported in GeTe-based thermoelectric, is realized in (Ge1- ySbyTe)1- x(Cu8GeSe6)x, primarily due to strong phonon scattering induced by point defects and precipitates. Simultaneously, Cu8GeSe6-alloying effectively suppresses the precipitation of Ge, enabling the optimization of carrier concentration with the additional help of aliovalent Sb doping. As a result, an extraordinary peak zT of up to 2.3 and an average zTavg. of ≈1.2 within 300-625 K are achieved, leading to a conversion efficiency of ≈9% at a temperature difference of 282 K. This work robustly demonstrates its potential as a promising component in thermoelectric generator utilizing low-grade waste heat.

Efficient rhombohedral GeTe thermoelectrics for low-grade heat recovery

Achieving high carrier mobility and thermoelectric performance in nearly twin-free rhombohedral GeTe (00l) films

Tuning the electronic structure of rhombohedral and cubic GeTe for thermoelectric application: Influence of molybdenum doping

Abstract GeTe is a very promising thermoelectric material, but the presence of massive intrinsic Ge vacancies leads to an overhigh hole concentration and poor thermal stability. Counter doping is commonly employed to reduce the hole concentration, which, however, unavoidably deteriorates the carrier mobility. Here, it is found that the intrinsic hole concentration in the rhombohedral phase is much lower than that in the cubic phase, owing to the higher formation energy of Ge vacancy in the former. With this recognition, the hole concentration of GeTe can be tuned to its optimum value simply by annealing below the phase transition temperature. As a result, “compositional plainification” is realized in the high‐performance GeTe‐based thermoelectrics with significantly reduced amounts of counter dopants and hetero‐alloys. A high carrier mobility of 150 cm2 V−1 s−1 is realized in GeTe at 300 K, which is much higher than that in conventional counter‐doped ones (≤60 cm2 V−1 s−1). More importantly, GeTe‐based compounds, with suppressed intrinsic vacancies, exhibit good thermal stability and reproducibility of thermoelectric performance. A high peak figure of merit, zT, of 2.14 at 670 K is obtained in Ge0.93Bi0.03Pb0.04Te. This work highlights the importance of understanding and regulating the intrinsic vacancy for high‐performance GeTe thermoelectrics.

Anomalous enhancement of thermoelectric performance in GeTe with specific interaxial angle and atomic displacement synergy

The GeTe compound has been revealed to be an outstanding thermoelectric compound, while its inherent high thermal conductivity restricts further improvement in its performance. Herein, we report a study on the synergistic optimization of the thermoelectric performance of GeTe by Bi-Se codoping. It is shown that the introduction of Bi decreases the carrier concentration and increases the structural parameter of the interaxial angle. With Se doping in the Te site, the lattice thermal conductivity is markedly reduced, while the carrier mobility is slightly influenced. Compared with the singly Se-doped GeTe, the Ge1-xBixTe1-ySey samples are more closed to a cubic phase, as indicated by the larger interaxial angle. On account of the reduction of carrier concentration and thermal conductivity, a ZTmax of 1.80 at 665 K and a high ZTave of 1.39 (400-800 K) are obtained in Ge0.94Bi0.06Te0.85Se0.15. This work reveals that the interaxial angle is vital to the performance optimization of rhombohedral GeTe.

Localized crystal imperfections coupled with phase diagram engineering yield high-performance rhombohedral GeTe thermoelectrics

Abstract GeTe alloys have attracted wide attention due to their high conversion efficiency. However, pristine GeTe possesses intrinsically massive Ge vacancies, leading to a very high hole concentration (1021 cm−3). Herein, a decreased carrier concentration is realized by alloying NaSbTe2 in GeTe due to the increased formation energy of Ge vacancies. This alloying also lowers energy separation between the valence bands in the rhombohedral GeTe and induces two extra valence band pockets around the Fermi surface along Γ‐L and L‐W in the cubic GeTe, all of which contributes to the higher power factors over a wide temperature range. Combined with the low lattice thermal conductivities due to plenty of dislocations and strains as a result of the crystallographic disorder of Na, Ge, and Sb, a maximum zT ≈ 2.35 at 773 K and a zTave of 1.33 from 300 to 773 K are achieved in (GeTe)90(NaSbTe2)10.

Electronic, optical, and thermoelectric properties of germanium tellurides (GeTe) were investigated through a series of first-principles calculations of band structures, absorption coefficients, and thermoelectric transport coefficients. We consider bulk GeTe to consist of cubic and rhombohedral phases, while the two-dimensional (2D) GeTe monolayers can form as a 2D puckered or buckled honeycomb crystals. All of the GeTe variants in the bulk and monolayer shapes are excellent light absorbers in a wide frequency range: (1) bulk cubic GeTe in the near-infrared regime, (2) bulk rhombohedral GeTe and puckered monolayer GeTe in the visible-light regime, and (3) buckled monolayer GeTe in the ultraviolet regime. We also found specifically that the buckled monolayer GeTe exhibits remarkable thermoelectric performance compared to the other GeTe phases due to a combination of electronic band convergence, a moderately wide band gap, and unique 2D density of states from the quantum confinement effect.

Developing high performance thermoelectrics to convert waste heat into useful electrical power is an important challenge that has the potential to impact our energy future. This requires exploring and discovering new materials with enhanced electronic and thermal (phonon) transport properties. Advances in computational modeling now allow the thermoelectric characteristics of materials to be calculated fully predictively, and thus represents a powerful tool to be partnered with experimental efforts.In this talk, I will present our recent findings on rhombohedral GeTe, a layered quasi-2D material predicted to have unusual anisotropic transport properties [1]. Our approach combines density functional theory with the Boltzmann transport equation, to extract the detailed electron and phonon dispersions along with the electron-phonon scattering rates, in order to predict the thermoelectric transport coefficients. Our results show that the electrons preferentially conduct along the cross-plane direction, while the holes favor the in-plane direction. Interestingly, this feature enhances the thermoelectric figure-of-merit ZT for n-type GeTe, since the conductivity (and power factor) is maximized perpendicular to the atomic layers while the lattice (phonon) thermal conductivity is minimized - leading to a three-fold increase in ZT. The cross-plane power factor and mobility in n-GeTe reach roughly 32 μW/cm-K2 and 500 cm2/V-s, respectively. An analysis of the band structure reveals that the large cross-plane transport originates from high-velocity conduction states, formed by the Ge p-orbitals that span across the interstitial region. These findings illustrate how the dominant electron and phonon transport directions are effectively decoupled in n-GeTe; a feature that can potentially be found in other materials to achieve enhancements in thermoelectric performance.[1] V. Askarpour and J. Maassen, “Unusual thermoelectric transport anisotropy in quasi-two-dimensional rhombohedral GeTe”, Phys. Rev. B 100, 075201 (2019). Figure 1

This article describes the preparation of GeTe-based alloy films using a solution-based technique. The dissolution behavior of GeTe was initially examined by comparing the weight loss of GeTe powder in different solvents, and it was found that, unlike in the cases of n-butylamine and NH₄OH, KOH fully dissolved GeTe to form an agglomerate-free solution. X-ray diffraction analysis revealed that the reaction between GeTe and KOH resulted in the formation of rhombohedral GeTe, cubic GeTe₄, and hexagonal Te structures after drying. GeTe-based alloy films were then prepared by the spin coating of the GeTe-containing solutions on a silicon substrate. The surface morphology and reflectance properties of the prepared films were found to be highly dependent on the spin speed, with optimization of the spin coating parameter resulting in the deposition of a continuous and smooth film.

Abstract High‐performance GeTe‐based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to low‐temperature rhombohedral GeTe, the high‐symmetry and high‐temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe‐based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe‐based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe‐based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe‐based thermoelectrics.

Electronic and elastic properties of rhombohedral GeTe: Exfoliation energy of a monolayer from (111) surface